Description will be given with reference to FIG. 7 and FIG. 8. FIG. 7 is a configuration drawing of a seat, and FIG. 8 is an exploded perspective view of a seat reclining apparatus for a vehicle of the related art.
As illustrated in FIG. 7, a seat 1 includes a seat cushion 2 configured to support buttocks of a seated person, and a seatback 3 provided so as to be tiltable in the fore-and-aft direction with respect to the seat cushion 2 and configured to support a back of the seated person. Reference numeral 4 denotes a reclining apparatus provided on an axis of rotation of the tilting movement of the seatback 3 and configured to allow/restrict the tilting movement of the seatback 3.
Subsequently, the reclining apparatus 4 will be described with reference to FIG. 8. In the drawing, the reclining apparatus 4 includes a ratchet (first member) 7 formed with inner teeth 7a extending along the circumferential direction of the circle and having an opened surface on one of the surfaces and a base plate (second member) 5 stacked on the ratchet 7 on the side of the opened surface and provided so as to be capable of rotating relative to the ratchet 7 in the circumferential direction.
The base plate 5 includes two pawls 10 each having outer teeth 10d capable of engaging the inner teeth 7a provided thereon so as to be capable of performing a rotational movement about a shaft portion 11 formed on the base plate 5.
Then, the respective pawls 10 are moved to a locked position where the outer tooth 10d engage the inner tooth 7a and an unlocked position where the engagement between the outer teeth 10d and the inner teeth 7a is released by a rotation cam 9. Furthermore, the respective pawls 10 are urged in the locked direction via the rotation cam 9 by a spring (urging member) 19 locked at one end thereof to the base plate 5 and the other end portion thereof to the rotation cams 9.
The rotation cam 9 is formed with a through hole 9a having serration on an inner wall surface at the center of rotation thereof. Then, the rotation cam 9 is operated from the outside of the reclining apparatus 4 by an operating shaft, not illustrated, penetrating through through holes 7b, 5a formed at centers of the base plate 5 and the ratchet 7, and fitted to the through hole 9a of the rotation cam 9.
The reclining apparatus 4 is provided with an unlock retaining mechanism including a curved guide 7d extending along the circumferential direction formed on the ratchet 7 on an inner cylindrical surface on the side of a bottom portion with respect to the inner teeth 7a thereof and projections 10b formed on the respective pawls 10 and configured to retain the pawls 10 in the unlocked position when abutting against the curved guide 7d. 
An annular spacer 23 configured to restrict the pawls 10 from moving to the bottom portion of the ratchet 7 is arranged on the bottom portion of the ratchet 7. The spacer 23 is press-fitted onto an outer peripheral surface of a cylindrical upright wall portion 7b formed on an inner wall surface of the ratchet 7 so as to project in the direction toward the base plate 5 along the peripheral edge of the inner tooth 7a. The upright wall portion 7b is formed by burring.
An outer peripheral portion of the ratchet 7 and an outer peripheral portion of the base plate 5 are held by a ring-shaped outer peripheral ring 8 and are integrated.
An operation of the reclining apparatus will be described. It is assumed that the ratchet 7 is provided on the seatback side, and the base plate 5 is provided on the seat cushion side.
Generally, the respective pawls 10 provided on the base plate 5 are in the locked position where the outer teeth 10d engage the inner teeth 7a of the ratchet 7 by an urging force of the spring 19, and hence a relative rotation between the ratchet 7 and the base plate 5 is restricted, and the seatback 3 is in the state of incapable of rotating with respect to the seat cushion 2 (a locked state).
When the operating shaft, not illustrated, is operated and the rotation cam 9 is rotated against the urging force of the spring 19, the respective pawls 10 move to the unlocked position where the engagement between the outer teeth 10d and the inner teeth 7a is released, and allows the relative rotation between the ratchet 7 and the base plate 5, so that the seatback 3 is allowed to rotate with respect to the seat cushion 2.
When an operating force to the rotation cam 9 is released, the outer teeth 10d of the respective pawls 10 provided on the base plate 5 engage the inner teeth 7a of the ratchet 7 by the urging force of the spring 19 and the relative rotation between the ratchet 7 and the base plate 5 is restricted, so that the locked state is achieved again.
As illustrated in FIG. 7, between a first level locked state A and a reclined state D, when the operating shaft, not illustrated, is operated and the rotation cam 9 is rotated to unlock and the operating force to the operating shaft is released, the pawls 10 are restored to the locked position by the urging force of the spring 19, so that the locked state is achieved.
In contrast, in a section between the first level locked state A and a forward folded state B (an unlock retaining state C), even when the operating force to the operating shaft is released by the above-described unlock retaining mechanism, the pawls 10 are retained in the unlocked position, so that the seatback 3 can be rotated without operating the operating shaft.